Mobile communications system refers generally to any telecommunications system which enables wireless communication when users are moving within the service area of the system. A typical mobile communications system is a Public Land Mobile Network (PLMN). Often the mobile communications network is an access network providing a user with wireless access to external networks, hosts, or services offered by specific service providers.
One of the main targets in the development of mobile communications networks is to provide the user with IP (Internet Protocol) service, i.e. access to the Internet through a mobile communication network. It is desired that the IP will be implemented as an overlay of the mobile network, while backwards compatibility with present systems is maintained with minimal modifications in the present standards. However, a problem is that the basic IP concept does not support user mobility: the IP addresses are assigned to network interfaces on the basis of their physical location. In fact, the first field of an IP address (the NETID) is common to all interfaces that are linked to the same Internet subnet. This scheme prevents the user (the mobile host) from keeping his/her address when moving in different Internet subnets, i.e. when changing the physical interface.
In order to enhance mobility in the Internet, a Mobile IP protocol for IP version 4 has been introduced by the Internet Engineering Task Force (IETF) in the standard RFC2002. The mobile IP enables the routing of IP datagrams to mobile hosts independently of the point of attachment in the subnetwork. The mobile IP protocol introduces the following new functional or architectural entities.
‘Mobile Node MN’ (also called Mobile Host MH) refers to a host that changes its point of attachment from one network or subnetwork to another. A mobile node may change its location without changing its IP address; it may continue to communicate with other Internet nodes at any location using its (permanent) IP address. ‘Mobile Station (MS)’ is a mobile node having a radio interface to the network. A ‘Tunnel’ is the path followed by a datagram when it is encapsulated. The encapsulated datagram is routed to a known decapsulation agent, which decapsulates the datagram and then correctly delivers it to its ultimate destination. Each mobile node is connected to a home agent over a unique tunnel, identified by a tunnel identifier which is unique to a given Foreign Agent/Home Agent pair.
‘Home Network’ is the IP network to which a user logically belongs. Physically, it can be e.g. a local area network (LAN) connected via a router to the Internet. ‘Home Address’ is an address that is assigned to a mobile node for an extended period of time. It may remain unchanged regardless of where the MN is attached to the Internet. Alternatively, it could be assigned from a pool of addresses.
‘Mobility Agent’ is either a home agent or a foreign agent. ‘Home Agent HA’ is a routing entity on a mobile node's home network which tunnels packets for delivery to the mobile node when it is away from home and maintains current location information for the mobile node. It tunnels datagrams for delivery to, and, optionally, detunnels datagrams from, a mobile node when the mobile node is away from home. ‘Foreign Agent FA’ refers to a routing entity in a mobile node's visited network which provides routing services to a registered mobile node, thus allowing the mobile node to utilise its home network address. The packets tunnelled by the mobile node's home agent are detunnelled and delivered to the mobile node by the foreign agent. For datagrams sent by the mobile node, the foreign agent may serve as a default router for registered mobile nodes.
RFC2002 defines ‘Care-of Address’ (COA) as the termination point of a tunnel toward a mobile node, for datagrams forwarded to the mobile node when it is away from home. The protocol can use two different types of care-of addresses: a “foreign agent care-of address” is an address announced by a foreign agent with which the mobile node is registered, and a “co-located care-of address” is an externally obtained local address which the mobile node has acquired in the network. An MN may have several COAs at the same time. An MN's COA is registered with its HA. The list of COAs is updated when the mobile node receives advertisements from foreign agents. If an advertisement expires, its entry or entries should be deleted from the list. One foreign agent can provide more than one COA in its advertisements. ‘Mobility Binding’ is the association of a home address with a care-of address, along with the remaining lifetime of that association. An MN registers its COA with its HA by sending a Registration Request. The HA replies with a Registration Reply and retains a binding for the MN.
The article “Performance Evaluation of Mobile IP Protocols in a Wireless Environment”, Maurizio Dell' Abate et al, IEEE International Conference on Communications, 1998, Conference Record, 1998, p. 1810-1816, discloses a Route Optimization MIP scheme which enhances the basic mobile IP (MIP). According to ROMIP, packets addressed to a particular host can be tunnelled directly from the source so that the intervention by the home agent is bypassed. The ROMIP allows every traffic source to cache and use binding copies. The original binding for a mobile host is kept in its home agent, but ROMIP supports a further update process in which the binding copy (current COA of the mobile host) is sent to the source host, normally in response to a first IP packet sent to a mobile host not located in the home network. The source host caches the received binding and uses it to tunnel the packet to a foreign agent FA indicated by the COA. When the destination mobile host suddenly moves to another subnet and under another foreign agent, the mobile host sends, immediately after it has moved, the binding update messages both to the home agent HA and the previous foreign agent FA. The source host has no way to become aware of the movement and it keeps sending IP packets to the old FA. These packets get lost until the old FA receives the above update. As soon as the old FA gets updated, it warns the source host and forwards incoming packets to the new FA. The handover ends when the source host, having received a fresh binding from the home agent HA, can tunnel its packets directly to the new FA.
The article “Mobile Internet Access and QoS Guarantees Using Mobile IP and RSVP with Location Registers”, Ravi Jain et al, IEEE International Conference on Communications, 1998, Conference Record, 1998, p. 1690-1695, discloses an alternative route optimization protocol, namely Mobile IP with Location Registers (MIP-LR). According to the MIP-LR, before launching a packet to a mobile host, a sending host first queries a database about the current location of the addressed mobile host. More particularly, when a mobile host moves from one subnet to another, it registers its current COA in a database called a Home Location Register (HLR). When a sending host has a packet to send, it first queries the HLR to obtain the mobile host's COA, and then sends packets directly to the mobile host. The location of the mobile host is also maintained in another database, a Visitor Location Register (VLR) in a visited subnetwork. Two mechanisms are proposed for updating the location of the mobile host in a cache of the sending host when the mobile host moves. In a first mechanism the mobile host informs the old VLR which traps any packets destined to the old COA and sends a binding warning message to the HLR. The HLR sends a binding update message containing the mobile host's new COA to the sending host. In a second mechanism the mobile host maintains a list of all the other hosts it has active connection with, and sends a binding update to each such host.
Neither of these prior art mobility management schemes is absolutely suitable for implementing IP mobility management in a mobile communication system which does not employ IP as a network protocol. The TETRA network, which tunnels IP traffic through a non-IP protocol, is an example of such a system. The TETRA system is a digital mobile communication system developed primarily for professional and governmental users, such as the police, the military, oil plants, etc. The mobility management problem will now be illustrated with reference to FIG. 1 which shows a TETRA network connected to the Internet. The TETRA network comprises digital exchanges DXT and TETRA base stations TBS. There are two possible configurations of how a DXT can be connected to the internet. In the first configuration each DXT unit may have its own direct “exit” via an adjacent router, such as router 1 for DXT1 and router 2 for DXT2 in FIG. 1, for forwarding IP packets from the TETRA network to the Internet and vice versa. In the second configuration only one or some of the DXT units, referred to as gateway DXTs herein, are connected to an Internet router (e.g. DXT1 to router 1 in FIG. 1), and the other DXTs are connected to the Internet over these gateway DXTs.
Internet routers see TETRA networks as ordinary local IP networks. Each TETRA network is assigned a set of unique IP addresses (IPv4 addresses, for example). The IPv4 address is composed of 32 bits and presented as a network and host identifier pair. The network identifier (netid) specifies a TETRA IP network and the host identifier specifies a mobile host in the TETRA network. In the TETRA network, an IP subnetwork can be formed around one or multiple DXTs. In the latter case, several DXTs will be logically organized under the same netid prefix and will share the same host address space. In the example illustrated in FIG. 1, a TETRA subnetwork 1 is formed around DXT1 and assigned a netid ‘192.1.1.0’, and a TETRA subnetwork 2 is formed around DXT2 and assigned a netid ‘192.1.2.0’. The organization of IP networks around DXTs requires routing capabilities on the links between them. Each single- or multi-DXT network must be able to forward datagrams destined to other TETRA IP networks. In FIG. 1 the DXT1 and the DXT2 are interconnected by a link 10.
The mobile hosts may move from one cell to another and thereby arbitrarily roam between the DXTs. The mobile host may start an IP data exchange in one network and complete it in another. The movement of the mobile hosts among TETRA networks leads to a situation where the netid identifier in the IP address of the mobile does not necessarily correspond to the current network. The IP address of the mobile host will not correspond to any TETRA network if the user has an IP subscription to another network than the TETRA. Further, the basic Internet routing protocols choose routes based on the destination network identifier but do not support mobility. If TETRA-networks utilize only standard internet routing protocols, IP datagrams will not reach roaming users.
The Mobile IP solves the mobility management problem of the basic IP by adapting to the inflexibility of the IP address, therefore it is not fully applicable to the TETRA network. Some of the mobile IP features unfavourable to the TETRA are listed below:                In the mobile IP, the mobile host is identified with respect to its home IP network, whereas in the TETRA the mobile user is identified by both an individual subscriber identity and a network service point identifier (NSAPI).        In the mobile IP, the mobile host is associated with its home network and dependent on the home agent. In the TETRA this would imply that the mobile host must be bound to a particular network. The dependence on that network reduces the robustness of the IP service as a whole. If the home agent is unreachable at the time when the mobile host, located in a visited network, acquires an IP address, the IP service will be denied. The availability and robustness of the service is especially essential in the TETRA which is used by the police, the fire service, etc., for emergency and command purposes.        In the mobile IP, the location of the mobile host is defined by the network identifier of the current IP network. In the TETRA this would mean that the movement of the mobile host can be detected only if the mobile host moves from one IP network to another. If there is an intention to reuse the existing TETRA infrastructure, then each DXT must be associated with a certain IP network identifier in order to bind the IP network numbers with the TETRA network topology. Each DXT must be equipped with TCP/IP stack and must participate in unnecessary IP tunneling that introduces an IP overhead in each tunneled datagram (a basic unit of information passed in an IP packet). We must also send, or broadcast, MIP messages enclosed in IP packets over the air interface. This will load a poor radio interface link and introduce a 40-byte overhead in each message. The tunneling could be justified only if the TETRA network would utilize Internet routing protocols anyway. The tunneling also means that the traffic flow towards the mobile host must be passed through the home agent making the IP service more vulnerable to a failure of the home agent.        In the mobile IP, the mobile host must carry out an authentication and location update procedure every time it moves to another network. In the TETRA this introduces an additional data exchange over the air interface. Further, the TETRA utilizes specific authentication and location update procedures.        
Also the enhanced ROMIP and MIP-LR protocols described in the above mentioned articles are unsuitable to the TETRA. The prior art methods have two features in common: 1) The protocols are designed for IP networks and adapted to the IP environment, consequently both protocols associate the user location with the IP network identifier, 2) The protocols handle mobility in an end-to-end manner, meaning that both communicating ends must take care of mobility management. Therefore, despite the optimizations introduced by the methods, all the inconsistent features which are described above with respect to the basic MIP are also valid for ROMIP and MIP-LR. Further, the ROMIP and MIP-LR can be applied only if both communicating ends support the ROMIP or MIP-LR protocol. Otherwise the communication falls back to the conventional MIP. A further major disadvantage in the ROMIP and MIP-LR relates to handover. The mobile host itself discovers a visited network after having received an advertisement from Foreign Agent. This introduces a certain idle period when none of the agents (Home Agent, old Foreign Agent and new Foreign Agent) knows the real location of the mobile host. During that period an appropriate agent discards datagrams destined to the mobile host.